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It depends on if it's really a backfire or a very loud bang.  Very loud bangs are explosions in the exhaust caused by being too rich and having unburnt gas hitting the hot exhaust.  Actual back firing and/or stumbling can be caused by many things.  You may need to lower your needles but it's hard to say without knowing the throttle opening.


The choke must be wide open at all times - open is pulling on the cable - all the way tight.  I eliminate the choke on every Amal concentric equipped bike I build.  Some will say it's needed for starting - in 54 years I've never had/used one.


Say what throttle opening rather than RPM that the backfires happen.  There are four basic phases of tuning for the carbs and depending on the gear you're in 2000 rpm would different throttle openings.


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